It's a study done by Boeing for a Next-generation The two winged RLV to replace the shuttle by 2010. This study was conducted in 2002. The flyback boosters would be 98% indentical. The stack would take off with all engines burning. At roughly the same time SRB's are jettisoned on the shuttle, the lower booster would detach and fly back to the Shuttle Landign Facility. Meanwhile, the other booster, full due to fuel crossfeed lines, flies into orbit with a mini-shuttle or cargo container on it's back.
Here's another picture: http://www.space.com/php/multimedia...ce+Launch+Initiative+reusable+launch+vehicle..